Lead iron tungstate capacitive transducer, relaxor material therefor and method of manufacture of said relaxor material

ABSTRACT

The present invention discloses a relaxor material lead iron tungstate which has been synthesized in doped and undoped conditions by single and two step heat treatment. The relaxor material is seen to exhibit almost negligible hysteresis and a transducer made thereby shows pressure measurement capability over a wide range from 0.5 MPa to 415 MPa with accuracy of ±0.05%.

This is a divisional of application Ser. No. 10/108,926 filed Mar. 29,2002; now U.S. Pat. No. 6,715,358 the above-noted application herebyincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to a lead iron tungstate capacitive transducer.More particularly, the invention relates to a lead iron tungstatecapacitive pressure transducer with low temperature coefficient, highpressure coefficient and low hysteresis.

BACKGROUND OF THE INVENTION

Measurement of pressure is very vital in industrial manufacturing andprocessing. Particularly measurement of pressure with accuracy over awide range is needed in such industries as automobiles, aerospace, steeland for synthesis of high strength materials. In all these industrialsectors, the accuracy in measurement is of paramount importance not onlydue to quality considerations but also to safety requirements. No singlegadget can measure the entire pressure range with the same accuracy andreproducibility. The gadgets may also not be sensitive enough to smallchanges in pressure and be stable over a wide working temperature (inthe range of 10-50° C.). A system is therefore required which will havethe necessary characteristics of large pressure coefficient to detectsmall changes even in a large absolute value and have a minimum driftover a large temperature range i.e. have a low temperature coefficient.

Pressure measurements have traditionally been made using a liquid columnmanometer. While this serves as an absolute instrument, its use islimited to lower ranges for pressure of 0.1 Pa to 200 kPa. Anotherdisadvantage of this device is that it cannot be transported easily fromone place to another. P. L. M. Heydemann and B. E. Welch, et al in‘Experimental Thermodynamics’, (Vol. II, B. LeNiendre and B. Vodar(eds), Butterworths (1975)), R. S. Dadson, et al in ‘The PressureBalance: Theory and Practice’, National Physical Laboratory, Teddington,England and J. K. N. Sharma and Kamlesh K. Jain, Pramana, J Phys Vol 27pp 417 (1986) disclose that pressures up to 300 MPa can be measuredeasily by piston gauges and that these piston gauges can be transportedafter taking certain precautions. However, these piston gauges cannot beused for pressures beyond 300 MPa without increasing the size of theentire assembly thereby making it cumbersome to use even with trainedmanpower. As a result this device is useless for fieldwork.

G. F. Molinar and L. Bianchi and J. K. N. Sharma, et al disclose the useof manganin resistance wires to sense pressures over a wide range. Themain drawback of manganin resistance wires is the low accuracy of just±0.1% when the requirement normally is of at least ±0.05% or better.Further this sensor has the undesirable property of zero shift with timeleading to erroneous measurements and needs stringent temperaturecontrol during measurement. While this device may be useful forhigh-pressure work, the use for low pressure ranges like 58 Mpa islimited. In order to cover lower ranges one necessarily has to useanother device.

Another pressure measuring device is disclosed by A. W. Birks (ReportNo. 1566 of Queen's University of Belfast). This disclosure describesthe device as a Strain Gauge. However, this device also suffers from thesame drawbacks as for manganin wire. Further the accuracy in pressuremeasurement of this device is low due to large hysteresis and zeroshift.

Yet another type of a pressure measuring device based on resistancemeasurement has been disclosed in a U.S. Pat. No. 5,578,765. The saidpatent disclosure teaches that the pressure due to an applied force on atransducer array, essentially consisting of resistive elements leads toa change in the resistance value when the applied force is changed. Thedependence of pressure is related to gradual touching of the two arraysthereby decreasing the resistance of the system. The inventors havedisclosed curvilinear relation between the measured resistance and theapplied pressure. At high pressures, the resistance drops to fairly lowvalues. This low resistance values may not be measurable so accuratelythereby leading to possible errors in pressure measurement. Anotherdrawback is that the device of this patent needs a threshold pressurefor it to act as a pressure sensor. As a result, the use of this deviceis limited in respect of pressures lower than the required thresholdvalue.

G. F. Molinar, et al in 1998 attempted to use a ceramic rod to improveupon the existing pressure sensor (Measurement, Vol. 24, pp 161 (1998)).While this pressure transducer had improved resolution and sensitivity,it lacked repeatability and had marked hysteresis. The presence of lastproperty is undesirable as this leads to increased error in the measuredpressure.

PCT Application PCT/WO US9405313 discloses a capacitive transducer thatcan measure pressure from as low as 100 PSI to 22,000 PSI. However, thestructure used is rather complicated—a metal diaphragm is separated froma dielectric alumina by as small a distance as 0.00005 inch and 0.020inch. This small distance between the metal diaphragm and the insulatordisc is difficult to maintain. Further the transducer when needed for afield experiment, does not possess the ruggedness to withstand transitmovements. The device of this disclosure also has high hysteresis due toits very structure.

Andeen, et al, in Rev. of Sci. Instruments, Vol. 42, PP 495, (1971),disclose the use of ionic crystals as pressure sensing elements whenformed as capacitor in sandwich structure. The pressure measurement isbased on the principle of change in capacitance with applied pressure,of the capacitor structure with the material as dielectric mediumbetween two electrodes. However the materials reported showed a largerchange in capacitance by a change in temperature (temp. coefficient=250ppm/° C.) and low pressure coefficient (−38 ppm/MPa). As a result, thematerials disclosed serve more as temperature sensors rather thanpressure sensors.

Kamlesh K. Jain and Subhash C. Kashyap in ‘High Temperature and HighPressures’ Vol. 27/28 pp 371 (1995), disclose the use of bismuthgermanium oxide. It is disclosed that pressure coefficient andtemperature coefficient of capacitance are 100 ppm/MPa and 60 ppm/° C.respectively. This is an indication of the utility of variation ofcapacitance with pressure as a means to measure pressure. Thereliability is guaranteed to a certain extent due to low temperaturecoefficient but not to a level of being used as a pressure gauge.

Yet another material has been disclosed by M. V. Radhika Rao, et al in JMaterial Science Letter Vol. 12, pp 122 (1997). The material disclosedis a relaxor material with the following composition: 44% Lead IronNiobate, 44% Lead Zirconium Niobate and 12% Barium Titanate. Thepressure coefficient of this complex was observed to increase butwithout any significant decrease in temperature coefficient therebyagain rendering the material not worthy of being used as a pressuretransducer with capacitance parameter. Typical sintering processparameter as temperature: 900° C. The pressure coefficient was 430ppm/MPa while temperature coefficient was +0.002/° C. Thus, the saidrelaxor material does not have much use as a pressure transducer.

The general draw back in all the prior art disclosure, is, therefore,low accuracy, limited usable pressure range, dependence on the need tomaintain precise temperature of the transducer, and hysteresis.

OBJECTS OF THE INVENTION

The main object of the present invention is to provide a lead irontungstate capacitive transducer.

Another object of the invention is to provide a process of preparationof lead iron tungstate material with low thermal coefficient,high-pressure coefficient and low hysteresis.

A further object of the invention is to provide a solid statecalcination method for preparation of doped lead iron tungstate relaxormaterial.

A still further object of the present invention is to provide a two stepcalcination process for the preparation of lead iron tungstate relaxormaterial not requiring any doping.

Another object of the invention is to provide a capacitance pressuretransducer for wide pressure range measurement from a low value of 0.5MPa to a high value of 415 MPa.

SUMMARY OF THE INVENTION

Accordingly the present invention provides lead iron tungstatecapacitive pressure transducer which comprises: a disc having a polishedsmooth first flat surface, a polished smooth second flat surface, thesaid polished smooth first flat surface being completely coated withmetal electrode, the polished smooth second flat surface also beingcoated with metal electrode, the said metal electrode on polished smoothsecond flat surface comprising formed coated circular portionscomprising a central portion and a coated annular concentric portionseparated from the central portion by an annular concentric clearregion, conducting metal wires being fixed to the metal electrode onpolished smooth first flat surface, metal electrode on coated centralportion of polished smooth second flat surface and metal electrode oncoated annular concentric portion of polished smooth second flatsurface.

In an embodiment of the present invention, the metal electrode isselected from the group consisting of silver, aluminum and gold.

In another embodiment of the present invention, the thickness of metalelectrode is in the range of 1000-2000 Å.

In another embodiment of the present invention, the width of annularconcentric region is in the range of 10-50 λ.

In yet another embodiment of the present invention, the metal wires areselected from gold and silver.

In another embodiment of present invention, purity of metal wire is atleast 99.99%.

In a further embodiment of the present invention, the metal electrode isdeposited by a known vacuum evaporation method such as thermalevaporation.

In a further embodiment of the present invention, the capacitivepressure transducer is useful for pressure measurement in a range of 0.5MPa-415 MPa.

In another embodiment of the present invention, the accuracy of pressuretransducer is ±0.05% over the entire range of 0.5 MPa-415 Mpa.

In yet another embodiment of the present invention, the absolute valueof pressure coefficient of the transducer is in the range of 497 ppm/MPato 622 ppm/MPa.

In another embodiment of the present invention, the temperaturecoefficient of the transducer is in the range of −0.006/° C. to 0.008/°C.

In another embodiment of the present invention, the transducer hasnegligible hysteresis.

The invention also relates to lead iron tungstate relaxor material usedfor manufacture of capacitive transducers comprising in undoped formstoichiometric Pb(Fe_(2/3) W_(1/3))O₃.

In one embodiment of the invention, the relaxor material is doped withlead in an amount of 1% by wt or 5% by weight.

The invention also relates to a process for the preparation of relaxormaterial useful in the manufacture of lead iron tungstate capacitivetransducer by subjecting appropriate mixture of weighed amount of thewet ground iron oxide and tungsten oxide and lead oxide taken in suchquantities so as to yield the final material as an undopedstoichiometric Pb(Fe_(2/3) W_(1/3))O₃ to solid state sintering.

In one embodiment of the process, the purity of the starting materialsis at least 99.9%.

In another embodiment of the invention, excess PbO is used to obtain aself-doped stoichiometric relaxor material the level of doping being tothe extent of 1% 5% by weight.

In another embodiment of the process, doping is done by adding excessamount of PbO salt in the initial mixture and wet grinding the mixtureso obtained.

In another embodiment of the invention, the wet ground material iscalcined at a temperature of at least 800° C. for a period of 2 hours.

In another embodiment of the invention, the calcined material is furtherground for about ten hours to ensure complete homogenization of themixed and reacted constituents.

In another embodiment of the invention, a binder, preferably polyvinylalcohol is added to the homogenized powder.

The invention also relates to a two-step calcination process for thepreparation of lead iron tungstate relaxor material by subjectingappropriate mixture of weighed amount of the wet ground iron oxide andtungsten oxide to calcination at a temperature of about 1000° C. for aperiod of 2 hours, subjecting the calcined material to further grindingfor about ten hours after mixing the lead oxide to yield a final productstoichiometric Pb(Fe_(2/3) W_(1/3))O₃.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

In the drawings accompanying this specification;

FIG. 1 represents variation of relative dielectric constant withpressure. Plot (A) is for pure lead iron tungstate material, (B) is for1 wt % lead doped material and (C) is for 5 wt % lead doped material.

FIG. 2 represents variation of relative dielectric constant withtemperature of the sample. Plot (A) is for pure lead iron tungstatematerial, (B) is for 1 wt % Pb doped material and (C) is for 5 wt % leaddoped material.

FIG. 3 represents variation of relative dielectric constant withpressure. Curve (A) is for the second calcination temperature of 750° C.Curve (B) is for the second calcination temperature of 810° C. and curve(C) is for the second calcination temperature of 830° C. The sampletemperature during capacitance measurement is 30° C.

FIG. 4 represents variation of relative dielectric constant withtemperature of the sample. Curve (A) is for the second calcinationtemperature of 750° C. Curve (B) is for the second calcinationtemperature of 810° C. and curve (C) is for second calcinationtemperature of 830° C. The applied pressure during all measurements was0.1 MPa.

DETAILED DESCRIPTION OF THE INVENTION

The relaxor material of the present invention is prepared by solid statesintering. All the starting materials are pure and preferably have apurity of at least 99.9%. The materials are weighed in such quantitiesso as to yield the final material as an undoped stoichiometricPb(Fe_(2/3) W_(1/3))O³, (PFW). The same material can also be prepared byusing excess PbO such that a self-doped stoichiometric PFW is obtained.Doping is done by putting excess amount of PbO salt in the initialmixture for wet grinding, for homogenization of the material.

Excess amount of lead oxide is added to compensate for any loss of thelead component due to high vapour pressure during high temperaturetreatment. The other advantage of adding excess lead oxide is to getself-doping of lead in the final material to see the effect on thecharacteristics.

The weighed and wet ground material is then calcined to effect thecomplete reaction of the oxides to form the PFW. The calcination isgenerally done at a temperature of at least 800° C. for a period of 2hours. The calcined material is further ground for about ten hours. Thislong duration grinding is necessary to ensure complete homogenization ofthe mixed and reacted constituents. A binder, preferably polyvinylalcohol was added to this powder. This mixture is then put in apelletising machine for making samples.

In a preferred embodiment, two-step calcination process for thepreparation of lead iron tungstate relaxor material is used. In thismethod, referred to as Columbite method, all the starting materials arepure and preferably have a purity of at least 99.9%. The materials areweighed in such quantities so as to yield the final material as astoichiometric Pb(Fe_(2/3) W_(1/3))O₃ herein after referred to as PFW.The appropriate weighed amount of the wet ground iron oxide and tungstenoxide is mixed and then calcined at a temperature of preferably at 1000°C. for a period of 2 hours. The calcined material is further ground forabout ten hours after mixing the lead oxide. This long duration grindingis necessary to ensure complete homogenization of the mixed and reactedconstituents. The mixed calcined powder is again calcined at atemperature in a range of 750 to 830° C. and preferably at a temperatureof 810° C. A binder, preferably polyvinyl alcohol is added to thispowder. This mixture is then put in a pelletising machine for makingdisc shaped samples.

Typical size of the samples in both the preferred embodiments ofpreparation of relaxor material was, but not limited to, 18 mm indiameter and 1.5 mm thickness. The PFW samples prepared were then usedto determine the parameters for pressure measurement. These samples werecoated with a silver film on both sides by vacuum evaporation tocomplete the capacitive structure. The electrode structure was such thatone flat surface of the disc was coated completely by a thin film of,preferably, silver. The other flat surface opposite to the first surfaceof the pellet was also coated with the silver film through a thin wirering mask such that a central circular portion of the coated film wasformed along with a peripheral annular concentric film at the rim. Allthe depositions were done by standard vacuum thermal evaporationsystems. The two portions were separated by a narrow clear annularconcentric space. Width of this clear annular space was typically 50 λ.The annular concentric ring was used to eliminate errors due to straycapacitance during ac measurements. Thin silver wires of purity 99.99%wee attached to the metal electrodes.

The so formed capacitive structure was then used to measure the thermalcoefficient and the pressure coefficient of the doped and undoped PFWmaterial prepared by the two preferred embodiments of this invention forthe preparation of relaxor material.

For temperature and pressure measurement, the capacitive structure wasplaced in a standard specimen holder. This holder was placed in aconventional high pressure vessel. Temperature of the vessel wasmaintained to within ±0.05° C. using temperature bath (Model No. RTE8DD, NESLAB, USA). Pressure was transmitted through diethyl hexylsebacate fluid. At a preset constant temperature, pressure was variedgradually from atmospheric pressure (0.1 MPa to 415 Mpa) and thevariation in the capacitance of the specimen was measured at fixedfrequency of 1 kHz by the automatic capacitance bridge (AndeenHagerling, model 2500 A, USA). During the measurement of pressurecharacteristics of the relaxor material, the data were taken ofvariation of capacitance with pressure increasing in magnitude as wellas with decreasing pressure from the maximum pressure applied. This wasdone to determine the hysteresis in the material.

FIG. 1 shows the variation of the ratio K/K₀ with applied pressure at asample temperature of 30° C. The ratio K/K₀ is determined by calculatingthe dielectric constants K and K₀ from the measured capacitance usingthe formula as given below:

${{Dielectric}\mspace{14mu}{constant}} = \frac{{Thickness}\mspace{14mu}{of}\mspace{14mu}{pellet} \times {Capacitance}}{{{Electrical}\mspace{14mu}{permitivity}\mspace{14mu}{of}\mspace{14mu}{vacuum} \times {Area}\mspace{14mu}{of}\mspace{14mu}{parallel}\mspace{20mu}{plate}}\mspace{14mu}}$

Here K is the dielectric constant with pressure applied and K₀ is thedielectric constant without any applied pressure.

In FIG. 1, plot (A) is the variation of K/K₀ with pressure for undopedrelaxor material and shows a near straight line without any hysteresis.Plot (B) in the same figure is for doped material with 1 wt % Pb. Theslope of this line is seen to be more than that of (A) indicating therole of doping in improving pressure characteristics. This is due to thefact that a small change in pressure results in a large change indielectric constant. Curve (C) is for a 5 wt % doped lead material whichfurther gives an enhanced slope of the curve between______ and pressure.Thus, increased doping leads to better characteristics of the material.The pressure coefficient being calculated by using the followingexpression:

${{Pressure}\mspace{14mu}{coefficient}} = \frac{{Change}\mspace{14mu}{in}\mspace{14mu}{dielectric}\mspace{14mu}{constant}}{{{Initial}\mspace{14mu}{Dielectric}\mspace{14mu}{Constant} \times {change}\mspace{14mu}{in}\mspace{14mu}{pressure}}\mspace{11mu}}$

Next keeping a fixed pressure say of 0.1 MPa, temperature was variedfrom 10° C. to 50° C. to measure the temperature coefficient ofcapacitance.

During the measurement of temperature characteristics of the relaxormaterial, data was taken of variation of capacitance with temperatureincreasing in magnitude as well as with decreasing temperature from themaximum temperature reached in order to determine the hysteresis in thematerial.

From the capacitance data and the dielectric constant, temperaturecoefficient and the pressure coefficient of the specimen were calculatedusing following formulas

${{Temperature}\mspace{14mu}{coefficient}} = \frac{{Change}\mspace{11mu}{in}\mspace{14mu}{the}\mspace{14mu}{dielectric}\mspace{14mu}{constant}}{{Initial}\mspace{14mu}{Dielectric}\mspace{14mu}{Constant} \times {change}\mspace{14mu}{in}\mspace{14mu}{temperature}}$

The dielectric constant was determined using the capacitance value andother material parameters and constants from the expressions givenearlier in the description.

FIG. 2 shows the variation of K/K₀ as a function of temperature at agiven fixed pressure, say 0.1 MPa. Curve (A) is for undoped materialwhile (B) and (C) are for 1 wt % and 5 wt % doped materialsrespectively. Plot (A) in the figure gives the slope of the variation ashigher than that for plot (B) and (C). This clearly indicates thatdoping by lead improves the temperature behavior of the lead irontungstate and that the material can be easily put to use as a pressuretransducer having the desired property of high pressure coefficient, andlow temperature coefficient.

FIG. 3 shows the variation of the ratio K/K₀ for the lead iron tungstaterelaxor material samples prepared with the two-step calcination(Columbite process), with applied pressure at a sample temperature of30° C., maintained to within ±0.05° C. The ratio K/K₀ is determined bycalculating the dielectric constants K and K₀ from the measuredcapacitance using the following formula:

${{Dielectric}\mspace{14mu}{constant}} = \frac{{Thickness}\mspace{14mu}{of}\mspace{14mu}{pellet} \times {Capacitance}}{{{Electrical}\mspace{14mu}{permitivity}\mspace{14mu}{of}\mspace{14mu}{vacuum} \times {Area}\mspace{14mu}{of}\mspace{14mu}{parallel}\mspace{20mu}{plate}}\mspace{14mu}}$

K is the dielectric constant with pressure applied and K₀ is thedielectric constant without any applied pressure.

In FIG. 3 plot (A) is the variation of K/K₀ with pressure for therelaxor material and shows a near straight line without any hysteresis.The plot is for a sample which was calcined for a second time at 750° C.after mixing required quantity of lead oxide for a stoichiometricmaterial. Plot (B) in the same figure is for material with secondcalcination temperature of 810° C. The slope of this line is seen to bea bit less than that of (A) indicating the role of increase in sinteringtemperature on the pressure characteristics. Curve (C) is for a samplewith second calcination temperature of 830° C., which shows someanomalous behaviour but has a tendency to give enhanced slope of thecurve between K/K₀ and pressure. This points to the fact that increasein cacination temperature may affect the pressure characteristics. Thepressure coefficient being calculated by using the following expression:

${{Pressure}\mspace{14mu}{coefficient}} = \frac{{Change}\mspace{14mu}{in}\mspace{14mu}{dielectric}\mspace{14mu}{constant}}{{{Initial}\mspace{14mu}{Dielectric}\mspace{14mu}{{Constant}.} \times {change}\mspace{14mu}{in}\mspace{14mu}{pressure}}\mspace{11mu}}$

Next keeping a fixed pressure say of 0.1 MPa, temperature of the samplewas varied from 10° C. to 50° C. to measure the temperature coefficientof capacitance.

During the measurement of temperature characteristics of the relaxormaterial, the data were taken of variation of capacitance withtemperature increasing in magnitude as well as with decreasingtemperature from the maximum temperature reached. This was done todetermine the hysteresis in the material.

From the capacitance data the dielectric constant, temperaturecoefficient of the specimen were calculated using following formula

${{Temperature}\mspace{14mu}{coefficient}} = \frac{{Change}\mspace{11mu}{in}\mspace{14mu}{the}\mspace{14mu}{dielectric}\mspace{14mu}{constant}}{{Initial}\mspace{14mu}{Dielectric}\mspace{14mu}{Constant} \times {change}\mspace{14mu}{in}\mspace{14mu}{temperature}}$

The dielectric constant was determined using the capacitance value andother material parameters and constants from the expressions givenearlier in the description.

FIG. 4 shows the variation of K/K₀, as a function of temperature at agiven fixed pressure, say 0.1 MPa. Here K₀ is the dielectric constant at10° C. Plot (A) is the variation of K/K₀ with temperature for therelaxor material and does not show any hysteresis. The plot is for asample, which was calcined for a second time at 750° C. after mixing therequired quantity of lead oxide for a stoichiometric material. The curveshows a negative slope and decreasing with increasing temperature. Thismeans that the material has a better temperature characteristic whenworked at slightly higher temperature. Plot (B) in the same figure isfor a material with second calcination temperature of 810° C. (B) in thefigure gives the slope of the variation as higher than that for plot (A)though still being negative in the temperature range studied. Curve (C)is for a sample with second calcination temperature of 830° C. Thiscurve shows an anomalous behavior compared to (A) and (B) but is stillcapable of being used as a pressure transducer. This is clearlyindicative of the fact that the present process which does not use anydoping material can be easily put to use as a pressure transducer havingthe desired property of high pressure coefficient and low temperaturecoefficient.

The above mentioned behavior in pressure and temperature characteristicsmay well be attributed to increase in grain size of the polycrystallinematerial which is formed as perovskite phase.

The scientific principle underlying the use of lead iron tungstaterelaxor material for pressure measurement lies in the fact that thesematerials show a large change in capacitance per unit change in appliedpressure. In other words these materials have a large pressurecoefficient of capacitance. Another characteristic of the material isthat it has a low value for temperature coefficient. This property isvery desirable and essential for the material to act as a pressuresensor usable in an environment where temperature fluctuations areinevitable. Also the material to be useful as a pressure sensor shouldnot have a memory effect i.e. should not have a hysteresis.

The novelty of the relaxor material of the present invention lies in itshaving low temperature coefficient, high pressure coefficient and lowhysteresis due to the inventive step of doping by lead in excess of 1%of lead to the parent lead iron tungstate material.

For preparing the Lead Iron Tungstate [Pb(Fe_(2/3) W_(1/3))O₃—specimensabbreviated as PFW], starting oxides were PbO, Fe₂O₃ and WO₃. Specimenswere prepared using following formulaPbO+1/3Fe₂O₃+1/3WO₃+Xwhere X is the excess (0%, 1%, 5%) wt. % of PbO. PFW was prepared as 7gm sample by taking 4.4171 gm of PbO, 1.0535 gm of Fe₂O₃ and 1.5294 gmof WO₃.

In the two-step Coulumbite process X was zero.

The following examples are given by way of illustration only and shouldnot be construed to limit the scope of the invention.

EXAMPLE-1

Weighed quantities of lead oxide, tungsten trioxide and ferric oxidewere taken and mixed and wet ground in acetone for 10 hours. Thismixture was then calcined at 810° C. for 2 h. The calcined powder wasfurther ground for 10 hours. To this ground calcined powder, polyvinylalcohol was added as binder, for making circular pellets of diameter 18mm and thickness 1.5 mm. The pellet was later sintered at a temperatureof 870° C. for 2 hours. After sintering the specimen was cooled andafter polishing of the surfaces, silver electrodes were formed on theflat surfaces by vacuum evaporation.

EXAMPLE 2

The material of Example1 was used to measure the pressurecharacteristics. The temperature of the material was kept constant at30° C. to within ±0.05° C. by keeping the material in a constanttemperature bath. The capacitance of the capacitor structureincorporating the lead iron tungstate material; was measured as afunction of pressure applied from 0.1 MPa to 415 MPa. The dielectricconstant of the material was then calculated and plotted as a functionof pressure. Pressure coefficient calculated from slope of variation ofdielectric constant with pressure was found to be −500 ppm/MPa

EXAMPLE 3

The material of Example 1 was used to measure temperaturecharacteristics. Pressure applied on the material was kept constant at100 Mpa. Capacitance of the capacitor structure incorporating the leadiron tungstate material was measured as a function of temperature of thematerial (varying from 10-50° C.) keeping the temperature constant towithin ±0.05° C. by keeping the material in a constant temperature bath.Dielectric constant of the material was then calculated and plotted as afunction of temperature. The temperature coefficient calculated from theslope of the variation of dielectric constant with temperature was foundto be −0.0066/° C.

EXAMPLE 4

Weighed quantities of lead oxide, tungsten trioxide and ferric oxidewere taken and mixed with additional amount of 1 wt % PbO and wet groundin acetone for 10 hours. This mixture was then calcined at 810° C. for 2h. The calcined powder was further ground for 10 hours. To this groundcalcined powder, polyvinyl alcohol was added as binder for makingcircular pellets of diameter 18 mm and thickness 1.5 mm. The pellet wasthen sintered at a temperature of 870° C. for 2 hours. After sintering,the specimen was cooled and the surfaces polished and silver electrodeswere formed by vacuum evaporation.

EXAMPLE 5

The material of Example 4 was used to measure pressure characteristics.Temperature of the material was kept constant at 30° C. to within ±0.05°C. by keeping the material in a constant temperature bath. Capacitanceof the capacitor structure incorporating the lead iron tungstatematerial was measured as a function of pressure applied from 0.1 MPa to415 MPa. Dielectric constant of the material was then calculated andplotted as a function of pressure. Pressure coefficient calculated fromslope of variation of dielectric constant with pressure was found to be515 ppn/Mpa.

EXAMPLE 6

The material of Example 4 was used to measure temperaturecharacteristics. Pressure applied on the material was kept constant at100 MPa Capacitance of the capacitor structure incorporating the leadiron tungstate material was measured as a function of temperature of thematerial (varying from 10-50° C.) keeping the temperature constant towithin ±0.05° C. by keeping the material in a constant temperature bath.Dielectric constant of the material was then calculated and plotted as afunction of temperature. Temperature coefficient calculated from slopeof variation of dielectric constant with temperature was found to be−0.0069/° C.

EXAMPLE 7

Weighed quantities of lead oxide, tungsten trioxide and ferric oxidewere taken and mixed with additional amount of 5 wt % PbO and wet groundin acetone for 10 hours. This mixture was then calcined at 810° C. for 2h. The calcined powder was further ground for 10 hours. To this groundcalcined powder, polyvinyl alcohol was added as binder for makingcircular pellets of diameter 18 mm and thickness 1.5 mm The pellet wasthen sintered at a temperature of 870° C. for 2 hours. After sintering,the specimen was cooled and the surfaces polished and silver electrodesformed by vacuum evaporation.

EXAMPLE 8

The material of Example 7 was used to measure pressure characteristics.Temperature of the material was kept constant at 30° C. to within ±0.05°C. by keeping the material in a constant temperature bath. Capacitanceof the capacitor structure incorporating the lead iron tungstatematerial was measured as a function of pressure applied from 0.1 MPa to415 MPa. Dielectric constant of the material was then calculated andplotted as a function of pressure. Pressure coefficient calculated fromslope of the variation of dielectric constant with pressure and wasfound to be 556 ppm/Mpa.

EXAMPLE 9

The material of Example 7 was used to measure temperaturecharacteristics. Pressure applied on the material was kept constant at0.1 Mpa Capacitance of the capacitor structure incorporating the leadiron tungstate material was measured as a function of temperature of thematerial (varying from 10-50° C.) keeping the temperature constant towithin ±0.05° C. by keeping the material in a constant temperature bathDielectric constant of the material was then calculated and plotted as afunction of temperature. Temperature coefficient calculated from slopeof variation of dielectric constant with temperature was found to be−0.007° C.

EXAMPLE 10

Weighed quantity of wet ground iron oxide and tungsten oxide wascalcined at a temperature of 1000° C. for a period of 2 hours. Thecalcined material was further ground for about ten hours after mixingthe lead oxide. This mixture was then calcined at 750° C. for 2 h. Thecalcined powder was further ground for 10 hours. To this ground calcinedpowder polyvinyl alcohol was added as binder for making cylindricalshaped specimen which was then sintered at a temperature of 870° C. for2 hours. After sintering, the specimen was cooled and after polishing ofthe surfaces, silver electrodes were formed by vacuum evaporation.

EXAMPLE 11

The material of Example 10 was used to measure pressure characteristics.Temperature of the material was kept constant at 30° C. to within ±0.05°C. by keeping material in a constant temperature bath. Capacitance ofthe capacitor structure incorporating the lead iron tungstate materialwas measured as a function of pressure applied from 0.5 MPa to 415 MPa.Dielectric constant of the material was then calculated and plotted as afunction of pressure. Pressure coefficient calculated from slope ofvariation of dielectric constant with pressure was found to be −497ppm/Mpa.

EXAMPLE 12

The material of Example 11 was used to measure temperaturecharacteristics. Pressure applied on the material was kept constant at0.1 Mpa Capacitance of the capacitor structure incorporating the leadiron tungstate material was measured as a function of temperature of thematerial (varying from 10-50° C.) keeping the temperature constant towithin ±0.05° C. by keeping the material in a constant temperature bath.Dielectric constant of the material was then calculated and plotted as afunction of temperature. Temperature coefficient calculated from slopeof variation of dielectric constant with temperature was found to be−0.0033° C.

EXAMPLE 13

Weighed quantity of the wet ground iron oxide and tungsten oxide wascalcined at a temperature of 1000° C. for a period of 2 hours. Thecalcined material was further ground for about ten hours after mixingthe lead oxide. This mixture was then calcined at 810° C. for 2 h. Thecalcined powder was further ground for 10 hours. To this ground calcinedpowder polyvinyl alcohol was added as binder for making cylindricalshaped specimen which was then sintered at a temperature of 870° C. for2 hours. After sintering, the specimen was cooled and after polishing ofthe surfaces, silver electrodes were formed by vacuum evaporation.

EXAMPLE 14

Material of Example 13 was used to measure pressure characteristics.Temperature of the material was kept constant at 30° C. to within±10.05° C. by keeping the material in a constant temperature bath.Capacitance of the capacitor structure incorporating the lead irontungstate material was measured as a function of pressure applied from0.5 MPa to 415 MPa. Dielectric constant of the material was thencalculated and plotted as a function of pressure. Pressure coefficientcalculated from slope of variation of dielectric constant with pressurewas found to be −534 ppm/Mpa.

EXAMPLE 15

The material of Example 13 was used to measure temperaturecharacteristics. Pressure applied on the material was kept constant at0.1 Mpa. Capacitance of the capacitor structure incorporating the leadiron tungstate material was measured as a function of temperature of thematerial (varying from 10-50° C.) keeping the temperature constant towithin ±0.05° C. by keeping the material in a constant temperature bathDielectric constant of the material was then calculated and plotted as afunction of temperature. Temperature coefficient calculated from slopeof variation of dielectric constant with temperature was found to be−0.008° C.

EXAMPLE 16

Weighed quantity of the wet ground iron oxide and tungsten oxide wascalcined at a temperature of at 1000° C. for a period of 2 hours. Thecalcined material was further ground for about ten hours after mixingthe lead oxide. This mixture was then calcined at 830° C. for 2 h. Thecalcined powder was further ground for 10 hours. To this ground calcinedpowder polyvinyl alcohol was added as binder for making cylindricalshaped specimen which was then sintered at a temperature of 870° C. for2 hours. After sintering, the specimen was cooled and after polishing ofthe surfaces, silver electrodes were formed by vacuum evaporation.

EXAMPLE 17

Material of Example 16 was used to measure pressure characteristics.Temperature of the material was kept constant at 30° C. to within ±0.05°C. by keeping the material in a constant temperature bath. Capacitanceof the capacitor structure incorporating the lead iron tungstatematerial was measured as a function of applied pressure from 0.1 MPa to415 MPa. Dielectric constant of the material was then calculated andplotted as a function of pressure. The pressure coefficient calculatedfrom slope of variation of dielectric constant with pressure was foundto be −622 ppm/Mpa.

EXAMPLE 18

The material of Example 16 was used to measure temperaturecharacteristics. Pressure applied on the material was kept constant at0.1 Mpa. Capacitance of the capacitor structure incorporating the leadiron tungstate material was measured as a function of temperature of thematerial (varying from 10-50° C.) keeping the temperature constant towithin ±0.05° C. by keeping the material in a constant temperature bathDielectric constant of the material was then calculated and plotted as afunction of temperature. Temperature coefficient calculated from slopeof variation of dielectric constant with temperature was found to be0.007° C.

The main advantages of the present invention are

-   1. The relaxor material can be used over a wide pressure range.-   2. The relaxor material can be used under varying temperature    ambiences thereby avoiding the use of additional means for    temperature control.-   3. The material can be used over a wide temperature range of 10-50°    C.-   4. The capacitive transducer can be used to measure pressure over a    wide range from 0.5 MPa to 415 Mpa with an accuracy of ±0.05% over    the entire range.

1. A two-step calcination process for the preparation of lead irontungstate relaxor material comprising subjecting appropriate initialmixture of starting materials of weighed amount of a wet ground ironoxide and tungsten oxide to calcination at a temperature of about 1000°C. for a period of 2 hours, subjecting the calcined material to furthergrinding for about ten hours after mixing lead oxide therein and thensubjecting the mixed calcined material again to calcination at atemperature in a range of 750 to 830° C. to yield a final productstoichiometric Pb(Fe_(2/3)W_(1/3))O₃, wherein excess PbO is used toobtain a self-doped stoichiometric relaxor material, and doping is doneby adding excess amount of PbO salt to the initial mixture and wetgrinding the mixture so obtained in order to homogenize the mixture soobtained.
 2. A process as claimed in claim 1 wherein the purity of thestarting materials is at least 99.9%.
 3. A process as claimed in claim 1wherein a binder comprising polyvinyl alcohol is added to thehomogenized mixture.